High pressure sodium vapor lamps are now well-known and widely used for street, roadway and area lighting applications. The basic lamp type is described in U.S. Pat. No. 3,248,590 -- Schmidt, 1966, "High Pressure Sodium Vapor Lamp", and generally comprises an outer vitreous envelope or jacket of glass within which is mounted a slender tubular ceramic arc tube. The ceramic envelope is made of a light-transmissive refractory oxide material resistant to sodium at high temperatures, suitably high density polycrystalline alumina or synthetic sapphire. The filling comprises sodium along with a rare gas to facilitate starting, and mercury for improved efficiency. The ends of the alumina tube are sealed by suitable closure members affording connection to the electrodes. The outer envelope is generally provided at one end with a screw base having shell and eyelet terminals to which the electrodes of the arc tube are connected.
Up to the present time high pressure sodium vapor lamps have been conventionally operated on 60 cycle alternating current by means of ballasts which limit the current to the lamp rating. In such operation, the light generated by the discharge is due almost exclusively to the excitation of the sodium atom through the self-reversal and broadening of the sodium D-line at 589 nanometers. The lamp efficacy is high, up to 130 lumens per watt depending upon lamp size, but the color temperature is low, from 2000.degree. to 2100.degree. Kelvin. While object colors in all portions of the spectrum are recognizable, those at the "cool" end such as violets, blues and to some extent greens are muted or grayed down. As a result, the lamp has not been generally acceptable for indoor applications, particularly where critical color discrimination is required.
More recently, the color temperature of high pressure sodium vapor lamps has been raised and their color rendition has been improved by going to pulse operation. The method is described and claimed in copending application Ser. No. 649,900 of Mitchell M. Osteen, filed Jan. 16, 1976, titled "Color Improvement of High Pressure Sodium Vapor Lamps by Pulsed Operation, " and assigned like this application. By utilizing pulse repetition rates in the sonic range from 500 to 2000 hertz and short duty cycles from 10 to 30%, the color temperature has been increased from the common value of 2050.degree. K to as high as 2700.degree. K.